The present invention relates generally to the field of spinal implant systems, and particularly systems that employ elongated spinal implants, such as rod and plates, connected at various locations along the spinal column. More particularly, the invention concerns a connection assembly that provides variable angle and variable height adjustability to the elongated spinal implant relative to a bone fastener engaged to the spine.
Several spinal fixation systems have been developed for use in correcting and stabilizing sections of the spinal column and facilitating spinal fusion. In one such system, a bendable elongated spinal implant, such as a rod, is longitudinally disposed adjacent the vertebral column and then secured to various vertebrae along the length of the column by way of a number of bone fasteners or fixation elements. A variety of bone fasteners can be utilized, such as hooks or bone screws, which are configured to engage specific portions of a vertebra.
An example of one such system is the TSRH(copyright) Spinal System of Sofamor Danek Group, Inc. In this system, various hooks and bone screws are engaged to a spinal rod by way of eyebolts. In early versions of the TSRH(copyright) Spinal System, the vertebral hooks and bone screws were attached to the spinal rod at a fixed orientation, usually projecting perpendicularly below the rod. At the time, the TSRH(copyright) Spinal System presented a significant advance over prior systems in its versatility, strength of fixation, and ease of implantation.
However, one drawback faced by the original TSPR(copyright) Spinal System, as well as the other prevalent fixation systems, was that a surgeon was required to make significant adjustments to the contour of the bendable rod so that the bone fasteners could solidly engage the vertebral bodies. What was needed, then, was a bone fastener that could be connected to the spinal rod at a variable angle. In order to address this need, the TSRH(copyright) Variable Angle Screw was developed, as described in U.S. Pat. No. 5,261,909. This Variable Angle Screw utilized the same TSRH(copyright) eyebolt to achieve a connection to a spinal rod. In addition, the Variable Angle system incorporated a washer that fit over the eyebolt, engaged the spinal rod within a groove in one surface of the washer, and provided a radially splined surface facing the bone fastener. The bone fastener had a complementary splined surface so that the fastener could be situated at variable angular orientations relative to the spinal rod. A nut threaded onto the post of the eyebolt clamped all the components together to complete the assembly.
The Variable Angle Screw system of the ""909 Patent presented a significant advance over prior rod-based implant systems. The system of the ""909 Patent was relatively compact and required a minimal number of parts yet was able to accomplish a solid fixation of the bone fastener to the rod at a wide range of angular orientations. One drawback of the system was that the eyebolt-nut combination required side-tightening of the nut to clamp the system together. This side-tightening aspect required a larger surgical site about the spine so that a wrench could be manipulated. To address this difficulty, a top-tightening assembly was developed as disclosed in U.S. Pat. No. 5,282,801. The clamp assembly depicted in the ""801 Patent replaced the eyebolt and nut with a clamp body having a T-bar against which the head of the variable angle bone fastener was clamped. In addition, while the original TSRH(copyright) System relied upon tightening a nut against the variable angle bone screw, the top-tightening approach of the ""801 Patent utilized a set screw that acted against the spinal rod to push the spinal rod into the interlocking washer, and ultimately against a complementary spline face of the variable angle screw. With this system, the variable angle capability was retained, while a top-tightening feature was added.
With the addition of the top-tightening capability, the more recent TSRH(copyright) Spinal System has provided surgeons with a great deal of flexibility in the placement and orientation of bone fasteners, such as hooks and screws, relative to a spinal rod. The Variable Angle components greatly reduce the need to manipulate and bend the spinal rod to conform to the patient""s anatomy. Even with the great improvements presented by the TSRH(copyright) Spinal System, a certain amount of shaping or contouring of the spinal rod has still been required. Specifically, the rod must be shaped so that at the point of attachment of the bone fastener, the rod is the same distance from the vertebral body as the splined or interdigitating portion of the bone fastener. This vertical or height alignment is necessary so that the variable angle components are properly aligned for accurate connection when the assembly is clamped together. Thus, the spinal surgeon still has to spend a certain amount of time shaping the spinal rods during the surgery so that the fixation system can be properly implanted.
In order to address this difficulty, later systems were developed that provided for a certain degree of vertical adjustability. By vertical or height adjustability, it is meant adjustment along the length of the bone fastener, Adjustment in this dimension allows the rod to be situated at varying distances from the spine, or oriented with a pre-set contour regardless of the location of fastener.
An adaptation of the original variable angle screw concept of the ""909 Patent is presented in U.S. Pat. No. 5,611,800. This system retained the yoke configuration of the bone screw in the ""909 Patent, but added a multifaceted connecting feature on both surfaces of the yoke. While the ""800 Patent system added height adjustability it did so at the cost of a more complicated connector structure with four specially machined interdigitating surfaces.
Another approach has been suggested in U.S. Pat. No. 5,643,263. The connection assembly in the ""263 Patent uses a Schanz-type bone screw rather than the yoke bone screw of the ""909 Patent.
Thus, the screw described in the ""263 Patent includes an elongated smooth shank portion. The connection assembly also adds a second washer disposed between the original washer and the smooth shank of the bone screw. The interdigitating feature exists between the contacting faces of the adjacent washers. The variable height is accomplished by a groove provided in the opposite surface of the additional washer that allows the connection assembly to slide along the shank of the bone screw until it is finally clamped together by a set screw.
While the connection assembly shown in the ""263 Patent goes a step further toward an easy-to-implant variable angle, variable height assembly, it too has left some room for improvement. For example, the connector assembly of the ""263 Patent requires an additional washer that adds to the number of components that must be manipulated by the surgeon. In addition, the added washer increases the lateral profile of the implant assembly. In certain regions of the spine, such as the thoracic and cervical regions, there is limited space transverse to the vertebral bodies. An optimal spinal implant system that has universal applicability should have as small a lateral profile as possible.
In order to address the unresolved detriments of prior implant systems, the present invention contemplates a variable angle/variable height connection assembly for a spinal implant system. In one embodiment of the invention, a connection assembly includes a first member or body that defines an opening through which an elongated spinal implant, such as a spinal rod, can extend. A second member is provided that includes a second opening through which extends an elongated stem or shank of a bone fastener, such as a bone screw or hook. In some embodiments, the two members are attached by a connecting means that allows the members to pivot relative to each other about a connection axis. This connecting means thus provides a mechanism for variable angular orientations of the bone fastener relative to the elongated spinal implant. The second opening in the second member adds the height adjustment capability.
In a further feature of the invention, an interface washer is disposed over both the first and the second member of the connection assembly. The interface washer includes a first face having a groove defined therein for engagement with the elongated spinal implant extending through the opening in the first member. The interface washer also includes an opposite second face that is directed toward the stem of the bone fastener. This second face of the interface washer includes an interface element defined thereon. The shank of the bone fastener also includes a complementary interface element facing the interface washer.
In one specific embodiment, the second face of the interface washer includes a radially splined surface about a central opening. The central opening provides means for the washer to be passed over and about the first and second members of the connection assembly. The interface element on the shank of the bone fastener in this embodiment includes a rib extending along the length of the fastener. The rib is configured to reside between the radial splines of the interface washer.
With these components, the bone fastener can be moved up and down through the opening of the second member. In addition, the second member can be rotated relative to the first member so that the connection assembly can accommodate various angular orientations of the bone fastener relative to the elongated spinal implant. In the preferred embodiment, the central opening of the interface washer is non-circular to mate with a complementary non-circular profile of the first member of the connection assembly. The second member is sized so that it can be freely rotated within the central opening of the interface washer.
When the first and second members of the connection assembly are properly oriented relative to each other and to the bone fastener, a set screw can be threaded into the first member. The set screw includes an engagement tip for contacting the spinal rod within the opening of the first member. As the set screw is driven further into the first member, the engagement tip urges the spinal rod toward the shank of the bone fastener. Continued tightening of the set screw increases the clamping force between the spinal rod, interface washer, stem of the bone fastener, and ultimately a contact end of the opening in the second member.
In one embodiment of the invention, the rib of the bone fastener projects externally from an essentially constant diameter stem. In a further embodiment, the stem of the bone fastener includes a truncated surface along at least a portion of its length so that the shank has a non-circular transverse cross section. The interface element, or rib, then projects from the truncated surface. In an alternative embodiment, one or both of the interface washer and shank of the bone fastener can include a raised pattern, such as a crosshatch, multi-faceted, or knurled pattern. In a further embodiment, one of the interface washer or shank of the bone fastener can include a raised pattern, while the other of the two components can have a surface configured to permit penetration of the raised pattern into the surface.
In certain embodiments of the invention, a connection assembly includes a one-piece body that defines one opening for receiving the elongated spinal implant and a second substantially perpendicularly oriented opening for receive the stem of the bone fastener. One or both of the openings can be configured to permit variable orientations of the elongated spinal implant within the first opening or the stem of the bone fastener within the second opening. In one specific embodiment, one or both of the openings is defined by a pair of channels that widen from a central portion of the opening outward toward the outer surface of the body.
In a further alternative embodiment, the elongated spinal implant can carry an interface element for engagement with an interface element on the intermediate washer. The intermediate washer can include a groove on its opposite surface to engage a smooth stem of the bone fastener. In this embodiment, the bone fastener maintains a fixed orientation relative to the interface washer, just as the spinal implant has a fixed orientation in the embodiments identified above. The bone fastener still retains its height adjustment capability. The variable angle adjustment is manifested between the spinal implant, or rod, and the interface washer.
In yet another embodiment, the elongated spinal implant, or rod, is clamped within the first body of the connection assembly by way of a second washer and a nut. In this embodiment, the connection assembly includes a first member defining an aperture for receiving the spinal implant, and a second member pivotably connected to the first member and defining an aperture for receiving a bone fastener. An interface washer is disposed between the spinal implant and the bone fastener in a manner implemented in other embodiments described above. With this embodiment, the first member can include a threaded stem projecting outward in a direction opposite the interface washer. A second washer is threaded over the first body so the spinal implant, or rod, is sandwiched between the interface washer and the second washer. An internally threaded nut is threaded onto the threaded stem to clamp the connection assembly together.
The bone fastener to be used with the connection assembly described above can have a stem of different configurations to permit the interlocking engagement with the interface washer. One configuration defines an elongated hex having an elongated projecting rib at each point of the hex. Another configuration defines a cylindrical stem with multiple elongated projecting ribs on its surface. Yet another configuration can incorporate four concave surfaces having a double elongated projecting rib where two surfaces abut. These projecting ribs serve as engaging elements that can complement the interlocking element on the interface washer described above.
In another alternative embodiment of the invention, a connection assembly includes a one-piece body that defines a spinal implant opening and a bone fastener opening. The two openings are substantially perpendicular and overlapping. The bone fastener opening has a width that affords variable angular orientations of the bone fastener. A wall of the bone fastener opening opposite the spinal implant opening defines an engaging surface. The stem of the bone fastener can be cylindrical or can have substantially flat surfaces configured with projecting ribs for interlocking with the engaging surface. A set screw is used as a means to urge the spinal implant to press against the bone fastener and the bone fastener against the engaging surface.
It is one object of the present invention to provide a spinal implant system that includes bone fasteners capable of achieving variable angular and height/vertical orientations relative to an elongated member spanning between locations along the spine. A further object is to provide this variable angle/variable height capability with a minimum number of components that must be manipulated by the surgeon within the surgical site.
One benefit of the invention is that the variable angle/variable height components can be easily clamped together to insure a solid fixation of the instrumented portion of the spine. A further benefit is achieved by features of the invention that permit top-tightening of the components within the patient.
These and other objects and benefits of the invention will be made clear upon consideration of the following written description and accompanying figures.